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Fluvial Facies Models
Preliminary Evaluation of Lithofacies Models for Meandering Alluvial Streams
Recently completed and ongoing studies of meandering streams disclose a complexity of sedimentary facies not found in standard fining-upward models and later models from the Endrick and Wabash Rivers. The following features are usually deemed diagnostic of low-sinuosity undivided or braided streams but typify many meandering streams: negligible mud, large facies changes over short distances laterally, vertical sequences that do not denote an upward decrease in flow regime, predominance of gravel (sometimes quite coarse), scarcity of cross-stratification, absence of natural levees, and sheetlike geometry of channel facies. Many other widely used criteria for meandering streams do not occur in all meandering streams or else can exist in non-meandering streams.
The ubiquitous assumption of a close correspondence between channel pattern and lithofacies in rivers presents additional complications to fluvial facies models. The morphology of many streams varies greatly from one short reach to another, can be drastically modified by great floods, or can be transitional between recognized types of channel pattern.
Five lithofacies classes of meandering streams are identified tentatively: muddy fine-grained streams, sand-bed streams with modest fine member, sand-bed streams without mud, graveliferous sand-bed streams, and streams with coarse gravel and little sand. The last three classes include vital features ordinarily deemed typical of non-meandering streams.
These considerations imply that the reconstruction of depositional environment of ancient fluvial sediments requires detailed knowledge of the three-dimensional lithofacies. Failure to appreciate this fact may be one reason why environmental interpretations of ancient fluvial sediments commonly are equivocal. A second important factor may be the present ignorance of the manner and potential of preservation of individual fluvial units when stacked upon one another.
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